"Did the Court of Appeals for the Federal Circuit err in restricting district courts' equitable discretion in evaluating patent unenforceability, contrary to this Court's precedent in Keystone Driller, Hazel- Atlas, and Precision Instrument, by applying a rigid test that (a) forecloses district courts from considering the entire circumstantial record; and (b) precludes district courts from granting equitable remedies where a patent applicant has violated the PTO's duty of candor."